1. Field of the Invention
The present invention generally relates to a method and apparatus for coating print media in an inkjet printer system. More particularly, the present invention relates to a method and apparatus wherein the surface energy of rollers and/or doctor blades within the coating apparatus are controlled relative to the surface energy of the coating liquid.
2. Background Art
Drop-on-demand ink jet printers use thermal energy to produce a vapor bubble in an ink-filled chamber to expel a droplet. A thermal energy generator or heating element, usually a resistor, is located in the chamber on a heater chip near a discharge nozzle A plurality of chambers, each provided with a single heating element, are provided in the printer's print head. The print head typically comprises the heater chip and a nozzle plate having a plurality of the discharge nozzles formed therein. The print head forms part of an ink jet print cartridge that also comprises an ink-filled container.
Ink jet printers have typically suffered from two major shortcomings. First, optical density of a printed image varies greatly with the print media or substrate being printed upon. Second, ink drying time is excessive on some media types.
Interaction between the ink and print media or substrate influences the performance of the ink jet printer Different media types behave differently with the ink and not all media types are well suited for ink jet printing. Accordingly, attempts have been made to apply a liquid coating to the media before printing that interacts with the ink to improve the quality of the resulting printed image. The ink may contain, for example, penetrants to improve dry time and binders to improve performance. The “precoating” liquids may contain materials that cause the ink to flocculate on the surface of the media, improving image quality Precoating liquids have previously been applied to the print media using a separate ink jet print head and by the use of a roll coating apparatus that directly contacts the print media prior to ink application. One roll coating apparatus and method of the prior art is shown and described in U.S. Pat. No. 6,183,079, assigned to Lexmark International, Inc.
Precoating systems of the prior art, however, suffer from several shortcomings. For example, ink jet precoating systems require that the precoating liquid have a sufficiently low viscosity to pass consistently through the print head. Such liquids typically have an undesirably long dry time and cause undesirable cockle and curl in the medium. Prior art roll precoating systems have not provided optimum control over the amount of precoating liquid applied to the print medium Because the roll coater typically remains in contact with the medium during stop-start printing, coat weight irregularity, often referred to as “banding,” has occurred in prior art roll coating systems. Severe banding may be aesthetically unacceptable and may disturb the interaction between the coating liquid and the ink
Banding frequently occurs when the rolls are stopped and the printer is depositing ink onto the substrate. During that time, coating remaining on the rolls may be absorbed by the substrate, resulting in a high coat weight at that location and a visible band.
Coat weight irregularity may also result from capillary wicking under and around the doctor blade that meters coating liquid onto a roller in the roll coating system. When the roll coating system is idle, excess coating liquid may be drawn under or around the doctor blade and accumulate downstream of the doctor blade When the coating system is restarted, that accumulated coating liquid is transferred through the system, frequently resulting in coat weight irregularity.
Accordingly, there is a need for an improved ink jet printer and a coating apparatus for such a printer that is capable of printing images uniformly on a wide variety of commercially available substrates, wherein ink drying time is minimized and printed image quality is maximized.